First, my playgroup is fairly competitive. So my choice of general is ok with them. Originally the deck idea was just to lock my opponents down while I take chunks of their life away but then I realized the awesome power of Sun Titan. Budget is not a restriction, so go nuts. So here's my pseudo-list

First off, you don't have enough legendaries(IMO) to really play Captain Sisay...at least not to her full potential. Also, you say that your playgroup is competitive, and your general's alright with them...what about Hokori? Kudos if they can handle that and Winter Orb, but there's a fine line between being competitive, and being 'that guy', I've found...

Anyway, you might want to consider stuff like Crucible of Worlds, allowing much brokeness with your knight.Om-Nom is never a bad choice in a green deck, if only for last second mana(and he's tutorable with Sisay). If you're worried about getting stuck with only green mana, try Farrelite Priest and others. Darksteel Plate and other stuff can help if you need to use Mageta the Lion's ability, keeping at least one creature around if you don't have Dauntless Escort out for whatever reason. Quest for Renewal is often overlooked for decks with a lot of tappers...dern shame, in my opinion. For further creature recursion, also consider Reya Dawnbringer(again, tutorable with Sisay) and Emeria, the Sky Ruin. Finally, your graveyard seems to be a bit of your lifeline...consider some shenanigans(yeah, I know Teeg would prevent it normally) to get stuff like Leyline of Sanctity out, to avoid getting bog-slapped.

Here's a guide I wrote to a recursion engine I like to run in mono-white, but I think it can be useful to you. Pick out the pieces you like, and use them to support a Reveillark combo. Saffi and green's creature tutors should make it easy to pull off, and the recursion engine itself makes you very strong in the late game in case your combo gets disrupted. Eternal Witness makes this recursion engine AMAZING.

RECURSION ENGINE

What mono white lacks in card draw, it can often make up for with its recursion engine. The idea is to play defense, wipe the board when needed, and over time your dead things will start to become more and more difficult to keep off the board. Here's how the engine fits together. The first card is what is doing the recurring, and the following cards are what it recurs.

If it helps, you can draw a web and connect all of these visually. It's pretty impressive. Artisan of Kozilek can be shuffled back in by the other eldrazi and recurred with Eye of Ugin, and if you want more recursion, Pulsemage Advocate is strong in some environments.